OK I've closed the Thumbnail Panel but it hasn't helped. Seems the only thing I can do is stop using a Google folder from with SC.
Might be worth some trial-and-error, if there is something "odd" about the attributes of the Google folder that SC (for some obscure reason) cannot work nicely with - though I can't guess at what that reason might be, unless it is (say) security access control/ownership settings that are at the root of the problem.
As a workaround, if you create a Junction
folder to the Google folder, and operate purely
on the Junction
folder when using SC, then you may find that the difficulty disappears (is avoided), because SC is not working directly on the Google folder.
Suck it and see?
However, if it is (say) security access control/ownership settings that are at the root of the problem, then the Junction folder may well inherit the same settings as the actual Google folder to which it relates.